The principal objective of the proposed research is to study the physiology and effect on visually-related behavior, of a cholinergic feedback loop in a central visual processing area of the brain. The isthmo-tectal network in the midbrain of a lower vertebrate, the leopard frog, will be used as a model system. The specific aims are: 1) Explore the behavioral changes caused by unilateral and bilateral ablation of nucleus isthmi. 2) Block isthmo-tectal input to determine the effect of loss of n. isthmi input on tectal electrical activity. 3) Investigate the long term behavioral effects of ablation of n. isthmi alone and in conjunction with ablation to other brain structures to determine the extent of recovery of visual function. 4) Explore the behavioral consequences of tectal application of cholinergic antagonists. The long-term objective of this study is to determine how visual information is processed in the central nervous system in order to further our understanding of vision.